1. Field of the Invention
This invention relates to optical devices and, in particular, liquid crystal optical devices.
2. Art Background
Liquid crystal based devices are being used, and are being contemplated for use, in a wide variety of applications. These applications include uses that depend on relatively high-speed changes, i.e., changes faster than 5 milliseconds, in the optical state of the device. Exemplary of devices benefiting from high-speed operation are liquid crystal optical switches and liquid crystal large area displays, i.e., displays with more than 100.times.100 elements. The required optical change is generally produced by a change in molecular orientation of the liquid crystal material and is readable through a variety of optical expedients. (See Applied Physics Letters, 22, T. D. Beard et al, page 19 (1973), or High Technology, G. Graft, pages 55-69 (May 1984) for a description of liquid crystal optical switch configurations, and Graft supra for a review article describing a variety of other useful liquid crystal device configurations.)
Ferroelectric materials are one class of liquid crystal compositions being explored for use in high-speed devices. (See Applied Physics Letters, 36, N. A. Clark and S. T. Lagerwall, page 899 (1980) and Applied Physics Letters, 41, M. A. Handschy and N. A. Clark, page 39 (1982) for a description of various ferroelectric liquid crystal device configurations.) The characteristics of the device depend strongly on the properties of the ferroelectric liquid crystal being employed. The attainment of bistability, i.e., two optically different stable states where the relaxation time from one state to another is greater than 30 minutes, requires the pitch, i.e., the shortest distance between end points of a single twist of the helix in the liquid crystal phase, of the ferroelectric liquid crystal material to be at least 50 percent of the device thickness. The operating speed of a ferroelectric liquid crystal based device is also dependent on various liquid crystal material properties. For example, the viscosity and spontaneous polarization of the liquid crystal material are the primary influences for a given applied field, determining the speed at which the liquid crystal molecules can be spatially transformed and thus the speed at which optical changes are producible. However, it appears from present reports that the spontaneous polarization varies much more widely than the viscosity of ferroelectric liquid crystal materials, and thus the spontaneous polarization predominantly controls speed.
Generally, for a given applied electric field, the higher the spontaneous polarization, the faster the speed. However, the magnitude of applied electric fields is generally limited by practical considerations. The possibility of electrical breakdown of the liquid crystal material typically limits electric fields to less than 40 volts/.mu.m. Similarly, typical switching electronics afford voltages yielding electric fields generally less than 36 volts/.mu.m. It is desirable to have a spontaneous polarization above 0.2.times.10.sup.-8 coul/cm.sup.2 so that applied electric fields less than 36 volts/.mu.m still afford device switching speeds faster than 5 milliseconds.
As discussed, in ferroelectric devices it is quite desirable to utilize a liquid crystal material having a long pitch and a high spontaneous polarization. However, individual ferroelectric materials with long pitch, e.g., greater than 10 .mu.m, essentially invariably have a correspondingly low spontaneous polarization. Thus, the possibility of high-speed operation of ferroelectric liquid crystal devices for nominal device thicknesses and applied electric fields has been an illusive goal. Only one specific mixture of ferroelectric materials, i.e., ##STR1## has been reported to produce large pitches. (See JEPT Letters, 33(10), Beresnev et al, page 536 (1981).) However, the materials are not suitable for device applications because they are unstable, i.e., exhibit unacceptable degradation, i.e., greater than a 10 percent change in the clearing point over a period of 1 month. Thus, the attributes required for a stable high-speed device that operates at nominal applied electric fields have not been realized.